A solid state imager can be viewed as being made up of a number of laterally offset pixels containing photosensitive regions. Lens arrays placed in registration with the pixels of a solid state imager are well known to the art and can take a variety of forms. Each lens concentrates incident light into an associated pixel of the imager in order to optimize device performance by increasing light sensitivity. Interline imagers, in which the photosensitive region (photodiode) occupies only part of each pixel, particularly benefit from lens arrays, as is well known in the art. FIG. 1 shows a single pixel 10 of a typical prior art interline imager in registration with a lens 22, a photodiode 14 formed in a semiconductor substrate 12, a gate electrode 16, and a light shield 18. The gate electrode 16 and the light shield 18 are typically isolated electrically from one another and from the semiconductor substrate 12 by isolation oxides, not shown. A lens array includes a plurality of lenses 22. The photodiode 14, the semiconductor substrate 12, the gate electrode 16, and the light shield 18 form a semiconductor portion 40 of the imager.
Lens arrays integral to the imager are commonly made by thermal deformation of photolithographically defined polymers, such as photoresist, as taught for example by Ishihara, U.S. Pat. No. 4,667,092 for the case of cylindrically shaped lens arrays, or Weiss, U.S. Pat. No. 4,694,185 for the case of rectangular lens arrays.
Referring again to FIG. 1, the device shown has a lens supporting layer 20 for offsetting the lens array from the photodiode 14 to maximize collection of light in the photodiode 14. Conventionally, the lens supporting layer 20 includes at least one organic or inorganic spacer layer to achieve some degree of planarization. A blocked light ray 30 that is not collected by the photodiode 14 is shown. Referring now to FIGS. 2A and 2B, the lens supporting layer 20 can include an upper spacer layer 20a and a lower spacer layer 20b. A color filter element 26, as shown in FIG. 2B, can be positioned between the upper spacer layer 20a and the lower spacer layer 20b for the manufacture of color imagers and can thus also be part of the lens supporting layer 20. The use and limitations of such planarization layers in optically active solid state devices is taught by McColgin, U.S. Pat. No. 4,553,153 for a polymerizable monomer. Color filter arrays, such as those described in Nomura, U.S. Pat. No. 5,321,249, are also typically coated from organic materials.
Referring again to FIGS. 2A and 2B, to form a lens array over the lens supporting layer 20, a coating, typically of photoresist, is exposed on the lens supporting layer 20 and developed to produce an array of rectangular resist islands 28. The resist pattern is then flood exposed to bleach the remaining photochemistry and subsequently heated until the resist material flows enough to form the convex lenses 22, yet not so much as to cause adjacent lenses to flow together, as taught in U.S. Pat. No. 4,694,185. Therefore, there must be some spacing between the lenses 22, and as a result, some light is lost.
To fully utilize the available light, all incident light rays desired to be associated with a given pixel must be directed through the associated aperture 24 in the light shield 18 and into or near the photodiode 14. Because the opening of the aperture 24 is typically made small in order to reduce smear, as is well known in the art, and because of the need for spacing between the lenses 22 as previously noted, it has not been possible to direct all light rays to the photodiode 14. Moreover, the apertures 24 in the light shields 18 are frequently rectangular in shape when viewed from above the imager, whereas the lenses 22 are frequently square, when viewed from above the imager, further reducing the number of rays directed to the photodiode 14. Therefore, the sensitivity of the imager is reduced to less than what it might otherwise be.
Various improvements in the structure of the lens array have been directed to increasing the light gathering efficiency by directing a greater portion of the rays into the photodiode 14. Enomoto, U.S. Pat. No. 5,321,297, and Nakai, U.S. Pat. No. 5,293,267, teach methods of forming lenses having different curvatures in the directions corresponding to the long and short dimensions of the apertures, thereby increasing the fraction of incident rays directed toward the photodiode. Yonemoto, U.S. Pat. No. 5,306,926, teaches the use of spacer layers and planarizing layers using materials with indices of refraction that assist the direction of rays to the photodiodes. Masegawa, U.S. Pat. No. 5,371,397, shows a variety of light beam dispersion structures, as well as the use of layers having deliberately adjusted indices of refraction to better collimate rays entering the photodiode, thereby reducing smear. Other improvement efforts have been directed to reducing or utilizing the space between the lenses. For example, Jech, U.S. Pat. No. 5,324,930, laterally offsets the lens array with respect to photodiodes so that some light falling in the gap between the lenses can be utilized. Revelli et al., commonly assigned U.S. application Ser. No. 08/369,235 filed Jan. 6, 1995, discloses techniques for making lenses with smaller gaps.
Despite these improvements, it is still not possible to direct all incident rays through the apertures 24 and into the photodiodes 14, and thus increased sensitivity of the imagers is still required. This is particularly the case for small pixels, due to the small size of the apertures 24 and to the comparatively larger fraction of area of the lens array occupied by the gaps between the lenses 22. Furthermore, the art has encountered difficulties in stability of the organic materials, which must be subjected to temperatures sufficient to flow the material, but which must remain substantially transparent. Mehra et al., U.S. Pat. No. 4,966,831, teaches methods of protecting the lenses from oxidation, but these methods require additional fabrication steps.
A further difficulty encountered in the art is that the choice of indices of common polymeric lens materials is limited to values not too different than that of glass, thereby making overcoating of the lenses with polymeric materials for protection and for packaging difficult, since the refraction of light rays depends on the difference of the indices of the materials at the lens surface. In addition, as typically practiced, the upper lens surfaces are not conducive to further device processing because they are not planar, and therefore, not optimal for subsequent photolithography.
Yet another difficulty encountered in the art is that polymeric lens materials cannot be heated excessively, for example, to temperatures common in device testing or in instrument sterilization, without changes in lens shape or optical absorption. U.S. Pat. No. 5,321,297 teaches the transfer of the shape of a polymeric lens array fabricated on an inorganic lens layer to that layer by means of an isotropic etch having the same etch rate for the polymeric lens array material as for the inorganic lens layer material. While this method can produce lens arrays that can be heated and for which the selection of material with a wider range of optical properties is possible, the etch is difficult to control in practice and is not widely used in the art. In addition, this method does not solve the loss of optical collection efficiency due to gaps between lenses.
Finally, the methods currently practiced suffer difficulty in process control for devices of different sizes and suffer some loss of optical efficiency because the methods of achieving planarity are imperfect The shape of upper and lower spacer layers 20a and 20b and color filter elements 26 in FIGS. 2A and 2B is difficult to control because of the topography of electrodes 16 and light shields 18 especially using a common process for devices of different lateral sizes.